a) Field of the Invention
The present invention relates to a spark plug for an internal combustion engine, especially to a spark plug assuring fail-free ignition over a long period of time.
b) Description of the Related Art
Since an air-fuel mixture is enriched from the starting of an internal combustion engine until the completion of warming-up, carbon formed as a result of combustion of the fuel may deposit in a large quantity together with the liquid fuel on a leg portion of an insulator of a conventional spark plug. The carbon thus deposited on the leg portion of the insulator may then be oriented under electric forces (impressed voltages), whereby a path of carbon may extend from a basal part of the leg portion, i.e., a ground side toward a free end of the leg portion and the insulation resistance of the insulator may hence be lowered. This may cause engine troubles. With a view toward removing carbon deposited as described above and also promoting the self-cleaning action of the insulator itself, the leg portion of the insulator is made longer to prevent the insulation resistance from being lowered, and/or a highly water-repellant material such as silicone oil is coated to the surface of the insulator and/or the inner wall of the metal shell so as to avoid the formation of water which induces the deposition of carbon.
When the leg portion of the insulator is made longer to facilitate the rise of the surface temperature of the insulator, the insulator can maintain insulation resistance against the deposition of carbon due to combustion of the fuel. The thus-lengthened leg portion is therefore effective for improving the smear resistance. However, the leg portion thus lengthened tends to induce pre-ignition. A limitation is also imposed on the length of the leg portion from the standpoint of heat resistance. In the case of a spark plug in which the surface of an insulator and the inner wall of a metal shell are coated with a highly water-repellant material such as silicone oil, the highly-water repellant material such as silicone oil is caused to gradually evaporate and its effect for the elimination of water and the like is lost in a short time when repeatedly exposed to hot combustion gas.